Thick steel plate and method for manufacturing the same

ABSTRACT

A thick steel plate having a chemical composition containing, by mass %, C: 0.200% or more and 0.350% or less, Si: 0.05% or more and 0.45% or less, Mn: 0.50% or more and 2.00% or less, P: 0.020% or less, S: 0.005% or less, Al: 0.005% or more and 0.100% or less, at least one of Cu, Ni, Cr, Mo, V, Nb, Ti, B, REM, Ca, and Mg, and Fe and inevitable impurities. The thick steel plate being such that CI, which is defined by a particular equation, is 40 or more. Additionally, the thick steel plate having a microstructure in which the area fraction of a bainite phase is 60% or more, the area fraction of Martensite-Austenite constituent is 5% or more and less than 20%, and the remaining constituent phases are at least one of a ferrite phase, a pearlite phase, and a martensite phase.

TECHNICAL FIELD

The present disclosure relates to a thick steel plate which can preferably be used for members of, for example, industrial machines and transporting and conveying devices which are required to have abrasion resistance against, for example, rock, sand, ore, and slurry materials and a method for manufacturing the steel plate.

BACKGROUND ART

The members of, for example, industrial machines such as power shovels, bulldozers, hoppers, buckets, and dump trucks and transporting and conveying devices such as steel pipes used for transporting slurry materials, which are used in the field sites of, for example, construction, civil engineering, and mines, are subjected to abrasion in use due to, for example, earth and sand.

Conventionally, it is known that there is an increase in the abrasion resistance of a steel material by increasing the hardness of the steel material. Therefore, to date, for example, steel materials whose hardness is increased by adding a large amount of alloy chemical elements have been used for some kinds of members which are required to have satisfactory abrasion resistance.

However, since it is known that increasing the hardness of steel materials in order to increase abrasion resistance is accompanied by a significant decrease in workability, there is a problem in that it is difficult to use high-hardness materials in applications in which it is necessary to perform work on the materials.

Therefore, there is a demand for a steel material excellent in terms of workability while maintaining excellent abrasion resistance. For example, Patent Literature 1 proposes a steel plate having a chemical composition containing, by mass %, C: 0.13% to 0.18%, appropriate amounts of Si, Mn, P, S, Al, B, and N, Cr: 0.5% to 2.0%, Mo: 0.03% to 0.3%, and Nb: 0.03% to 0.1%, in which the constituent chemical elements satisfy the condition that HI is 0.7 or more, in which Ceq is more than 0.50, and in which HB is 360 or more and 440 or less at a temperature of 25° C. Here, HI=[C]+0.59[Si]−0.58[Mn]+0.29[Cr]+0.39[Mo]+2.11([Nb]−0.02)−0.72[Ti]+0.56[V], and Ceq=[C]+[Si]/24+[Mn]/6+[Ni]/40+[Cr]/5+[Mo]/4+[V]/14, where the atomic symbols respectively denote the contents (mass %) of the corresponding alloy chemical elements.

Patent Literature 1 describes that, according to the technique described above, by forming a martensite structure having a HB of about 400 by performing a quenching treatment, and by increasing the amount of a solid solution Nb, it is possible to increase high-temperature abrasion resistance.

Patent Literature 2 proposes a steel plate having a chemical composition containing, by mass %, C: 0.10% to 0.45%, appropriate amounts of Si, Mn, P, S, and N, and Ti: 0.10% to 1.0%, in which the number of TiC precipitates or compound precipitates of TiC with TiN and TiS having a grain diameter of 0.5 μm or more is 400 or more per 1 mm², and in which Ti*, which is expressed by a particular relational expression, is 0.05% or more and less than 0.4%.

Patent Literature 3 proposes an abrasion-resistant steel plate excellent in terms of workability, the steel plate having a chemical composition containing, by mass %, C: 0.05% to 0.35%, appropriate amounts of Si, Mn, and Al, Ti: 0.1% to 1.2%, in which DI*, which is expressed by a particular relational expression, is less than 60, and a microstructure including a ferrite phase-bainite phase structure as a matrix structure, in which hard phases are dispersed.

Patent Literature 2 and Patent Literature 3 describe that, according to the techniques described above, by forming precipitates mainly including TiC having a large grain diameter in a solidification process, it is possible to increase abrasion resistance at low cost.

CITATION LIST Patent Literature

PTL 1: Japanese Patent No. 4590012

PTL 2: Japanese Patent No. 3089882

PTL 3: Japanese Unexamined Patent Application Publication No. 2010-222682

SUMMARY Technical Problem

However, in the case of the technique according to Patent Literature 1, it is difficult to say that good workability is achieved, because martensite structure is formed by performing a quenching process, which results in a high hardness of HB360 or more. In addition, in the case of the technique according to Patent Literature 1, since a large amount of alloy chemical elements is added, there is an increase in alloy costs.

In the case of the techniques according to Patent Literature 2 and Patent Literature 3, there is an increase in manufacturing costs, because, since TiC having a large grain diameter is formed in a solidification process, it is necessary to repair the surface of the slab before rolling is performed. In addition, it is not clear whether high-temperature abrasion resistance is achieved by using the techniques according to Patent Literature 2 and Patent Literature 3.

Therefore, an object of the present disclosure is to provide an inexpensive thick steel plate excellent in terms of workability and abrasion resistance and a method for manufacturing the steel plate.

Solution to Problem

The present inventors, in order to achieve the object described above, diligently conducted investigations regarding the influence of various factors on abrasion resistance, and, as a result, found that, by optimizing the chemical composition of a steel material, by controlling a value which is defined as the total content of plural alloy chemical elements in the chemical composition to be a certain value, and by forming a steel microstructure in which the area fraction of a bainite phase is 60% or more, the area fraction of Martensite-Austenite constituent (hereafter referred to as ‘MA constituent’) in the bainite phase is 5% or more and less than 20%, and the balance is one, two, or all of a ferrite phase, a pearlite phase, and a martensite phase, it is possible to provide a steel material with excellent abrasion resistance while maintaining good workability without excessively increasing the hardness of the steel material.

The present disclosure has been completed on the basis of the knowledge described above and additional investigations. That is, the exemplary disclosed embodiments are as follows.

[1] A thick steel plate excellent in terms of abrasion resistance, the thick steel plate having a chemical composition containing, by mass %,

C: 0.200% or more and 0.350% or less, Si: 0.05% or more and 0.45% or less, Mn: 0.50% or more and 2.00% or less, P: 0.020% or less, S: 0.005% or less, Al: 0.005% or more and 0.100% or less, and the balance being Fe and inevitable impurities, in which CI, which is defined by equation (1) below, satisfies the condition that CI is 40 or more, and a steel microstructure in which the area fraction of a bainite phase is 60% or more, the area fraction of MA constituent in the bainite phase is 5% or more and less than 20% with respect to the whole microstructure, and the remaining constituent phases are one, or two or more of a ferrite phase, a pearlite phase, and a martensite phase.

CI=60C+8Si+22Mn+10(Cu+Ni)+14Cr+21Mo+15V   (1)

In the equation, atomic symbols respectively denote the contents (mass %) of the corresponding alloy chemical elements. However, the content of a chemical element which is not contained is set to be 0.

[2] The thick steel plate excellent in terms of abrasion resistance according to item [1], the thick steel plate having the chemical composition further containing, by mass %, one or more selected from

Cu: 0.03% or more and 1.00% or less, Ni: 0.03% or more and 2.00% or less, Cr: 0.05% or more and 2.00% or less, Mo: 0.05% or more and 1.00% or less, V: 0.005% or more and 0.100% or less, Nb: 0.005% or more and 0.100% or less, Ti: 0.005% or more and 0.100% or less, and B: 0.0003% or more and 0.0030% or less.

[3] The thick steel plate excellent in terms of abrasion resistance according to item [1] or [2], the thick steel plate having the chemical composition further containing, by mass %, one or more selected from

REM: 0.0005% or more and 0.0080% or less, Ca: 0.0005% or more and 0.0050% or less, and Mg: 0.0005% or more and 0.0050% or less.

[4] A method for manufacturing a thick steel plate excellent in terms of abrasion resistance, the method including:

heating a cast piece or a steel piece having the chemical composition according to any one of items [1] to [3] to a temperature of 950° C. or higher and 1250° C. or lower, performing hot rolling with a finishing delivery temperature equal to or higher than Ar_(a), and performing accelerated cooling immediately after the hot rolling has been performed, at a cooling rate of 5° C./sec or more to a temperature range of 400° C. or higher and 650° C. or lower.

[5] A method for manufacturing a thick steel plate excellent in terms of abrasion resistance, the method including:

heating a cast piece or a steel piece having the chemical composition according to any one of items [1] to [3] to a temperature of 950° C. or higher and 1250° C. or lower, performing hot rolling, performing air cooling to a temperature lower than 400° C., then performing reheating to a temperature equal to or higher than the Ac₃ and 950° C. or lower, and performing cooling immediately after the reheating has been performed, at a cooling rate of 5° C./sec or more to a temperature range of 400° C. or higher and 650° C. or lower.

Advantageous Effects

According to the present disclosure, it is possible to easily and stably manufacture an abrasion-resistant steel plate excellent in terms of workability and stably having excellent abrasion resistance, which has a marked effect on the industry.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating an abrasion test machine.

DESCRIPTION OF EMBODIMENTS

In the present disclosure, a chemical composition and a steel microstructure are specified.

[Chemical Composition]

In the description, % refers to mass %.

C: 0.200% or more and 0.350% or less

C is a chemical element which contributes to the formation of MA constituent and which is important for achieving excellent abrasion resistance. In the case where the C content is less than 0.200%, it is not possible to sufficiently realize the effects described above. On the other hand, in the case where the C content is more than 0.350%, there is a decrease in weldability and workability. Therefore, the C content is limited to be 0.200% or more and 0.350% or less, or preferably 0.210% or more and 0.300% or less.

Si: 0.05% or more and 0.45% or less

Si is an effective chemical element which functions as a deoxidizing agent for molten steel and which has a function of contributing to the formation of MA constituent by increasing hardenability. In order to realize such effects, the Si content is set to be 0.05% or more. On the other hand, in the case where the Si content is more than 0.45%, there is a decrease in weldability. Therefore, the Si content is limited to be 0.05% or more and 0.45% or less, or preferably 0.15% or more and 0.40% or less.

Mn: 0.50% or more and 2.00% or less

Mn is an effective chemical element which has a function of contributing to the formation of MA constituent by increasing hardenability. In order to realize such an effect, it is necessary that the Mn content be 0.50% or more. On the other hand, in the case where the Mn content is more than 2.00%, there is a decrease in weldability, and a large amount of MnS, which becomes the starting point at which fracturing occurs when work such as bending is performed, is formed. Therefore, the Mn content is limited to be 0.50% or more and 2.00% or less, or preferably 0.60% or more and 1.70% or less.

P: 0.020% or less

In the case where the P content in steel is large, there is a decrease in toughness. Therefore, it is preferable that the P content be as small as possible. In the present disclosure, it is acceptable that the P content be 0.020% or less. Therefore, the P content is limited to be 0.020% or less. Here, since excessively decreasing the P content causes an increase in refining costs, it is preferable that the P content be 0.005% or more.

S: 0.005% or less

In the case where the S content in steel is large, since S is precipitated in the form of MnS, there is a decrease in toughness, and MnS becomes the starting point at which fracturing occurs when work is performed. Therefore, it is preferable that the S content be as small as possible. In the present disclosure, it is acceptable that the S content be 0.005% or less. Therefore, the S content is limited to be 0.005% or less. Here, since excessively decreasing the S content causes an increase in refining costs, it is preferable that the S content be 0.0005% or more.

Al: 0.005% or more and 0.100% or less

Al is an effective chemical element which functions as a deoxidizing agent for molten steel. In order to realize such an effect, it is necessary that the Al content be 0.005% or more. In the case where the Al content is less than 0.005%, it is not possible to sufficiently realize such an effect. On the other hand, in the case where the Al content is more than 0.100%, there is a decrease in weldability and toughness. Therefore, the Al content is limited to be 0.005% or more and 0.100% or less, or preferably 0.015% or more and 0.040% or less.

CI=60C+8Si+22Mn+10(Cu+Ni)+14Cr+21Mo+15V≧40

In the equation, atomic symbols respectively denote the contents (mass %) of the corresponding alloy chemical elements, and the content of a chemical element which is not contained is set to be 0.

In the case where CI is less than 40, since the steel microstructure described above is not formed due to insufficient quench hardenability, it is not possible to achieve good abrasion resistance. Therefore, CI is limited to be 40 or more, or preferably 44 or more. In addition, in the case where CI is excessively large, since there is an excessive increase in quench hardenability, there is a case where the steel microstructure described above is not formed due to an increase in the amount of martensite formed. Therefore, it is preferable that CI be 80 or less, or more preferably 75 or less.

The chemical composition described above is the basic chemical composition, and the balance is Fe and inevitable impurities. In the disclosed embodiments, in order to improve properties, one, or two or more selected from among Cu, Ni, Cr, Mo, V, Nb, Ti, B, REM, Ca, and Mg may be added as selective chemical elements.

Cu: 0.03% or more and 1.00% or less

Cu is a chemical element which has an effect of contributing to the formation of MA constituent by increasing quench hardenability. In order to realize such an effect, it is necessary that the Cu content be 0.03% or more. On the other hand, in the case where the Cu content is more than 1.00%, there is a decrease in hot workability, and there is an increase in manufacturing costs. Therefore, in the case where Cu is added, it is preferable that the Cu content be limited to be 0.03% or more and 1.00% or less. Here, it is more preferable that the Cu content be limited to be 0.03% or more and 0.50% or less from the viewpoint of inhibiting a decrease in hot workability and of decreasing cost.

Ni: 0.03% or more and 2.00% or less

Ni is a chemical element which increases quench hardenability and which contributes to an increase in low-temperature toughness. In order to realize such effects, it is necessary that the Ni content be 0.03% or more. On the other hand, in the case where the Ni content is more than 2.00%, there is an increase in manufacturing costs. Therefore, in the case where Ni is added, it is preferable that the Ni content be limited to be 0.03% or more and 2.00% or less. Here, it is more preferable that the Ni content be limited to be 0.03% or more and 0.50% or less from the viewpoint of decreasing cost.

Cr: 0.05% or more and 2.00% or less

Cr is a chemical element which has an effect of contributing to the formation of MA constituent by increasing quench hardenability. In order to realize such an effect, it is necessary that the Cr content be 0.05% or more. On the other hand, in the case where the Cr content is more than 2.00%, there is a decrease in weldability, and there is an increase in manufacturing costs. Therefore, in the case where Cr is added, the Cr content is limited to be 0.05% or more and 2.00% or less, preferably 0.07% or more and 1.50% or less, or more preferably 0.20% or more and 1.00% or less.

Mo: 0.05% or more and 1.00% or less

Mo is a chemical element which has an effect of contributing to the formation of MA constituent by increasing quench hardenability. In order to realize such an effect, it is necessary that the Mo content be 0.05% or more. On the other hand, in the case where the Mo content is more than 1.00%, there is a decrease in weldability, and there is an increase in manufacturing costs. Therefore, in the case where Mo is added, the Mo content is limited to be 0.05% or more and 1.00% or less, preferably 0.10% or more and 0.80% or less, or more preferably 0.20% or more and 0.50% or less.

V: 0.005% or more and 0.100% or less

V is a chemical element which increases quench hardenability and which contributes to an increase in toughness through the effect of decreasing the grain diameter of a microstructure as a result of being precipitated in the form of carbonitrides. In order to realize such effects, it is necessary that the V content be 0.005% or more. On the other hand, in the case where the V content is more than 0.100%, there is a decrease in weldability. Therefore, in the case where V is added, the V content is limited to be 0.005% or more and 0.100% or less.

Nb: 0.005% or more and 0.100% or less

Nb is a chemical element which effectively contributes to an increase in toughness through the effect of decreasing the grain diameter of a microstructure as a result of being precipitated in the form of carbonitrides. In order to realize such an effect, it is necessary that the Nb content be 0.005% or more. On the other hand, in the case where the Nb content is more than 0.100%, there is a decrease in weldability. Therefore, in the case where Nb is added, the Nb content is limited to be 0.005% or more and 0.100% or less. Here, it is preferable that the Nb content be 0.010% or more and 0.030% or less from the viewpoint of decreasing the grain diameter of a microstructure.

Ti: 0.005% or more and 0.100% or less

Ti is a chemical element which contributes to an increase in toughness through fixing of solid solution N as a result of being precipitated in the form of TiN. In order to realize such an effect, it is necessary that the Ti content be 0.005% or more. On the other hand, in the case where the Ti content is more than 0.100%, since carbonitrides having a large grain diameter are precipitated, there is a decrease in toughness. Therefore, in the case where Ti is added, the Ti content is limited to be 0.005% or more and 0.100% or less. Here, it is preferable that the Ti content be limited to be 0.005% or more and 0.030% or less from the viewpoint of decreasing cost.

B: 0.0003% or more and 0.0030% or less

B is a chemical element which contributes to an increase in quench hardenability when added in small amounts. In order to realize such an effect, it is necessary that the B content be 0.0003% or more. On the other hand, in the case where the B content is more than 0.0030%, there is a decrease in toughness. Therefore, in the case where B is added, the B content is limited to be 0.0003% or more and 0.0030% or less.

REM: 0.0005% or more and 0.0080% or less

REM inhibits a decrease in toughness and the formation of MnS, which causes fracturing when work is performed, by fixing S. In order to realize such effects, it is necessary that the REM content be 0.0005% or more. On the other hand, in the case where the REM content is more than 0.0080%, since there is an increase in the amount of inclusions in steel, there is a decrease in toughness. Therefore, in the case where REM is added, the REM content is limited to be 0.0005% or more and 0.0080% or less, and preferably 0.0005% or more and 0.0020% or less.

Ca: 0.0005% or more and 0.0050% or less

Ca inhibits a decrease in toughness and the formation of MnS, which causes fracturing when work is performed, by fixing S. In order to realize such effects, it is necessary that the Ca content be 0.0005% or more. On the other hand, in the case where the Ca content is more than 0.0050%, since there is an increase in the amount of inclusions in steel, there is a decrease in toughness. Therefore, in the case where Ca is added, the Ca content is limited to be 0.0005% or more and 0.0050% or less, or preferably 0.0005% or more and 0.0030% or less.

Mg: 0.0005% or more and 0.0050% or less

Mg inhibits a decrease in toughness and the formation of MnS, which causes fracturing when work is performed, by fixing S. In order to realize such effects, it is necessary that the Mg content be 0.0005% or more. On the other hand, in the case where the Mg content is more than 0.0050%, since there is an increase in the amount of inclusions in steel, there is a decrease in toughness. Therefore, in the case where Mg is added, it is preferable that the Mg content be limited to be 0.0005% or more and 0.0050% or less, and more preferably 0.0005% or more and 0.0040% or less.

[Steel Microstructure]

A steel microstructure including a bainite phase in an amount of 60% or more in terms of area fraction (also referred to as area ratio), MA constituent in the bainite phase in an amount of of 5% or more and less than 20% in terms of area fraction with respect to the whole microstructure, and the balance being one, or two or more of a ferrite phase, a pearlite phase, and a martensite phase is formed. By controlling the phase fractions as described above, there is an increase in the plastic deformation capability of a steel plate, which results in good workability. In addition, it is possible to achieve excellent abrasion resistance without excessively increasing the hardness of the steel plate.

Bainite phase; 60% or more in terms of area fraction

In the case where the area fraction of a bainite phase is less than 60%, it is not possible to achieve the desired abrasion resistance or the desired workability. Therefore the content of a bainite phase is set to be 60% or more, and preferably 80% or more, in terms of area fraction.

MA constituent: 5% or more and less than 20% in terms of area fraction

Since MA constituent finely disperses in a bainite phase and has a high hardness, MA constituent contributes to an increase in abrasion resistance. In the case where the area fraction of MA constituent is less than 5% with respect to the whole microstructure, it is not possible to achieve the desired abrasion resistance. On the other hand, in the case where the area fraction described above is 20% or more, the effect of increasing abrasion resistance becomes saturated, and there is an excessive increase in the hardness of a steel plate, which results in a decrease in workability and toughness. Therefore, the area fraction described above is set to be 5% or more and less than 20%. Here, since MA constituent is formed between the laths of a bainite phase or at the grain boundaries of a bainite phase, and has a small grain diameter, it is difficult to distinguish between a bainite phase and MA constituent by using an optical microscope. Therefore, MA constituent is seen as a part of a bainite phase. That is, in the calculation of the above-described area fraction of the bainite phase, the area of MA constituent is included in the area of the bainite phase. However, the area fraction of MA constituent is calculated with respect to the whole microstructure.

The remaining constituent phases of the steel microstructure other than a bainite phase are one, or two or more of a ferrite phase, a pearlite phase, and a martensite phase.

Hereafter, a method for manufacturing the thick steel plate according to the present disclosure will be described.

In the case where a steel material having the chemical composition described above has the specified temperature after casting has been performed, the steel material is subjected to hot rolling without cooling the steel material or after having first cooled and then heated the steel material in order to obtain a steel plate having specified dimensions and shape. Although it is not necessary to impose particular limitations on what method is used for manufacturing a steel material, it is preferable that molten steel be prepared by using a known casting method such as one using a converter and that the molten steel be made into a slab having specified dimensions by using a known method such as a continuous casting method. An ingot casting-slabbing method may also be used in order to obtain a slab.

The slab heating temperature is limited to be 950° C. or higher and 1250° C. or lower. In the case where the heating temperature is lower than 950° C., since there is an excessive increase in rolling load due to an increase in deformation resistance, there is a decrease in rolling efficiency. In addition, in order to stably achieve satisfactory abrasion resistance, it is necessary to uniformly form MA constituent across the whole steel plate. In the case where the heating temperature is lower than 950° C., since there is insufficient diffusion of segregated chemical elements such as C and Mn existing in a micro-segregation portion in a steel material, MA constituent is preferentially formed in the segregation portion, which results in an uneven distribution of MA constituent. On the other hand, in the case where the heating temperature is higher than 1250° C., since an excessive amount of scale is formed, there is a decrease in yield ratio, and there is an increase in energy consumption. Therefore, the heating temperature is limited to be 950° C. or higher and 1250° C. or lower. Here, “slab heating temperature” refers to an average temperature in the thickness direction of the slab derived by thermal transfer-thermal conduction calculation. The average temperature in the thickness direction of a slab is almost equal to the temperature at a position located at ¼ of the thickness.

Hot rolling is performed with a finishing delivery temperature equal to or higher than Ar₃. In the case where the finishing delivery temperature is lower than the Ar₃, since ferrite is formed, a sufficient amount of bainite is not formed. Therefore, the finishing delivery temperature is set to be equal to or higher than the Ar₃. In addition, in the case where the finishing delivery temperature is excessively high, since austenite grains grow, there is an increase in austenite grain diameter. Therefore, since there is an excessive increase in the amount of martensite formed due to an excessive increase in quench hardenability, it is difficult to form the desired microstructure. Therefore, it is preferable that the upper limit of the finishing delivery temperature be 930° C. or lower. Here, it is possible to determine the Ar₃ transformation temperature from a thermal expansion curve obtained when cooling is performed from a temperature range for forming austenite. In addition, “finishing delivery temperature” refers to the surface temperature of a steel plate.

Accelerated cooling is started immediately after the hot rolling has been performed. “Immediately” means “within 30 seconds” after the hot rolling has been performed. The cooling rate is set to be 5° C./sec or more, and the cooling stop temperature is set to be 400° C. or higher and 650° C. or lower. In the case where the cooling rate is less than 5° C./sec, since ferrite is formed, a sufficient amount of bainite is not formed. Therefore, the cooling rate is set to be 5° C./sec or more. In addition, although there is no particular limitation on the upper limit of the cooling rate, since the upper limit of the cooling rate of accelerated cooling is dependent on thermal transfer at the surface of the steel plate, practical cooling rate is 80° C./sec or less. Here, “cooling rate” refers to an average cooling rate at a position located at ¼ of the thickness between the time accelerated cooling is started and the time accelerated cooling is stopped. In the present disclosure, the cooling start temperature, the cooling rate, the cooling stop temperature are specified in terms of the temperature at a position located at ¼ of the thickness, because it is considered that the temperature at a position located at ¼ of the thickness represent a temperature intermediate between that of the surface of the steel plate and that at a position at ½ of the thickness of the steel plate, and represents the average temperature of the whole thickness of the steel plate.

In the case where the cooling stop temperature is lower than 400° C., since bainite transformation is completed, a sufficient amount of MA constituent is not formed. On the other hand, in the case where the cooling stop temperature is higher than 650° C., since C is expended by pearlite formed when air cooling is further continued, a sufficient amount of MA constituent is not formed. Therefore, the cooling stop temperature is set to be 400° C. or higher and 650° C. or lower. Here, “cooling stop temperature” refers to the temperature at a position located at ¼ of the thickness when accelerated cooling is stopped.

Instead of performing an accelerated cooling process after hot rolling has been performed, the accelerated cooling process may be performed after a process in which radiation cooling is performed after hot rolling has been performed to a temperature lower than 400° C. in terms of the temperature at a position located at ¼ of the thickness at which ferrite transformation or bainite transformation is completed and in which reheating is then performed to a temperature equal to or higher than Ac₃ and 950° C. or lower. It is necessary that the accelerated cooling process be started before the temperature of the steel plate is lowered and ferrite transformation begins. Therefore, it is preferable that the accelerated cooling process be started within 30 seconds after the steel plate has been brought out of a reheating furnace.

In the case where the reheating temperature is lower than Ac₃, reverse transformation from ferrite to austenite does not sufficiently occur. Since it is necessary that the microstructure of the whole steel plate be transformed into austenite in the reheating process, reheating is performed to a temperature equal to or higher than the Ac₃ in terms of the temperature at a position located at ½t of the steel plate. In the case where the reheating temperature is higher than 950° C., there is a negative effect on toughness due to an increase in austenite grain diameter, and there is an increase in energy consumption. Therefore, the reheating temperature is set to be equal to or higher than the Ac₃ and 950° C. or lower. “Reheating temperature” refers to the temperature at a position located at ½t of a steel plate, and the reheating temperature is derived by thermal transfer-thermal conduction calculation. Here, it is possible to determine the Ac₃ transformation temperature from a thermal expansion curve obtained when heating is performed from a temperature range for forming ferrite to a temperature range for forming austenite.

EXAMPLES

By preparing molten steels having the chemical compositions given in Table 1 by using a vacuum melting furnace, and by casting the molten steel into a casting mold, 150 kg of steel ingots (slabs) were manufactured. The obtained slabs were heated and subjected to hot rolling, and then accelerated cooling was performed. Here, some of the steel plates were cooled with air after hot rolling had been performed, further reheated, and then subjected to accelerated cooling.

By taking test pieces from the obtained steel plates, microstructure observation and an abrasion test were performed. The testing methods are as follows.

(1) Microstructure observation

By taking a test piece for microstructure observation from a position located at ¼ of the thickness of the obtained steel plate so that the observation surface is a cross section parallel to the rolling direction, by then performing mirror polishing on the surface, and by performing nital etching, the microstructure was exposed. Subsequently, by observing three fields of view selected at random by using an optical microscope at a magnification of 400 times in order to obtain photographs, and by identifying a bainite phase through a visual test, an area ratio (bainite phase fraction) was calculated. Moreover, by performing mirror polishing again on the same test piece for microstructure observation, and by performing two-step etching, MA constituent was exposed. Subsequently, and by observing ten fields of view in a portion in which a bainite structure was formed by using a scanning electron microscope at a magnification of 2,000 times in order to obtain photographs, the area ratio of MA constituent (MA constituent phase fraction) was calculated by using image analysis software. Here, “area ratios” of a bainite phase and MA constituent refer to area ratios with respect to the whole microstructure.

(2) Abrasion Test

By taking an abrasion test piece (having a thickness of 10 mm, a width of 25 mm, and a length of 75 mm) from the obtained steel plate so that a position located at 0.5 mm from the surface of the steel plate was a testing surface (abrasion surface), and by mounting the test piece on an abrasion test machine illustrated in FIG. 1, an abrasion test was performed.

The abrasion test piece was mounted in a direction at a right angle to the rotational axis of the rotor of the abrasion test machine so that the surface of 25 mm×75 mm faces in the tangential direction of the circumference of the rotational circle, and then an abrasion material was loaded into the drum. Silica stone having an average grain diameter of 30 mm was used as an abrasion material.

The test was performed by rotating the rotor and the drum respectively at rotational speeds of 600 rpm and 45 rpm. After having rotating the rotor 10,000 times in total, the test was finished. After the test had been performed, the weight of each test piece was determined. By calculating the difference (=decrease in weight) between the weight after the test had been performed and the initial weight, and by using the decrease in weight of SS400 (JIS G 3101 “Rolled steels for general structure”) as a standard value, an abrasion resistance ratio ((standard value)/(decrease in weight of the test piece)) was calculated. A case where the abrasion resistance ratio was 1.5 or more was judged as the case of “excellent abrasion resistance”.

(3) Bending Workability

A 180-degree bending test was performed on a steel sample (having a width of 100 mm, a length of 300 mm, and the thickness of the original steel plate (t mm)) by using a pressing bend method with a bending radius of 2.0t (t: thickness) in accordance with JIS Z 2248 (2006). By performing a visual test, a case where a defect such as a crack or other was not found in the sample after the bending test had been performed was judged as the case of good bending workability.

The results of the tests described above are given along with the manufacturing conditions in Table 2. In the case of the examples of the present disclosure, that is, Nos. 1 through 15, 17, 18, and 20, the abrasion resistance ratio was 1.5 or more, which clarifies that these examples had excellent abrasion resistance. On the other hand, in the case of the comparative example No. 16 where the bainite phase fraction and the MA constituent phase fraction in the steel microstructure did not satisfy the requirements of the present disclosure, bending workability was poor. In addition, in the case of the comparative example No. 19 where the bainite phase fraction and the MA constituent phase fraction in the steel microstructure did not satisfy the requirements of the present disclosure, abrasion resistance was poor. In the case of Nos. 21 through 23 where the MA constituent phase fraction in the steel microstructure did not satisfy the requirements of the present invention, abrasion resistance was poor.

TABLE 1 (mass %) Steel No. C Si Mn P S Al Cr Mo Cu Ni V Nb A 0.212 0.38 1.12 0.009 0.0025 0.025 B 0.269 0.35 1.21 0.010 0.0023 0.030 C 0.337 0.41 1.16 0.008 0.0026 0.030 D 0.254 0.25 0.61 0.009 0.0023 0.026 0.87 E 0.287 0.35 1.75 0.011 0.0030 0.029 0.36 0.23 0.041 F 0.310 0.25 1.01 0.009 0.0025 0.024 0.70 0.09 0.023 G 0.223 0.30 1.86 0.007 0.0022 0.033 0.36 0.34 H 0.310 0.21 0.78 0.012 0.0021 0.022 0.51 0.16 0.09 0.08 0.067 0.021 I 0.221 0.31 0.73 0.008 0.0021 0.026 0.73 0.019 J 0.314 0.36 0.82 0.008 0.0025 0.024 0.56 0.018 K 0.287 0.32 0.99 0.008 0.0021 0.029 0.52 0.21 0.025 0.021 L 0.214 0.29 0.81 0.007 0.0026 0.024 0.47 0.13 0.022 M 0.231 0.36 0.93 0.008 0.0030 0.021 0.75 0.27 0.040 0.017 N 0.291 0.27 1.29 0.007 0.0020 0.036 0.10 0.14 0.023 O 0.279 0.38 0.69 0.007 0.0020 0.031 0.38 0.16 0.022 P 0.158 0.34 1.31 0.011 0.0025 0.034 0.17 0.21 0.014 Q 0.297 0.41 0.48 0.013 0.0031 0.036 0.13 0.26 0.18 0.081 0.019 R 0.243 0.29 0.89 0.013 0.0031 0.036 0.13 0.05 0.023 (mass %) Steel Ac3 Ar3 No. Ti B REM Ca Mg Cl (° C.) (° C.) Class A 40.4 835 755 Example B 45.6 805 730 Example C 49.0 777 713 Example D 42.8 814 769 Example E 69.0 797 657 Example F 0.0031 54.5 783 715 Example G 0.014 0.0013 63.7 798 666 Example H 0.010 0.0011 0.0041 0.0016 50.6 796 725 Example I 0.012 0.0009 42.0 832 772 Example J 0.011 0.0008 47.5 789 739 Example K 0.015 0.0011 53.6 808 717 Example L 0.013 0.0010 42.3 839 761 Example M 0.015 0.0011 54.0 842 731 Example N 0.015 0.0012 52.3 794 704 Example O 0.011 0.0011 43.6 818 750 Example P 45.5 850 749 Comparative Example Q 42.0 823 747 Comparative Example R 39.4 821 758 Comparative Example Chemical composition is expressed in the units of mass %. Note 1: An underlined portion indicates a value out of the range according to the present invention. Note 2: Cl = 60C + 8Si + 22Mn + 10(Cu + Ni) + 14Cr + 21Mo + 15V (the content of each chemical element is expressed in the units of mass %)

TABLE 2 Accelerated Cooling Rolling Accelerated Finish Cooling Heating Treatment Heating Rolling Cooling Stop Reheating Sample Steel Ac3 Ar3 Thickness Temperature Temperature Rate Temperature Temperature No. No. (° C.) (° C.) (mm) (° C.) (° C.) (° C./s) (° C.) (° C.) 1 A 835 755 15 1120 910 51 480 — 2 B 805 730 20 1180 880 42 450 — 3 C 777 713 25 1100 850 29 530 — 4 D 814 769 35 1050 910 21 620 — 5 E 797 657 18 1120 820 45 520 — 6 F 783 715 15 1140 870 50 500 — 7 G 798 666 20 1180 890 40 430 — 8 H 796 725 25 1080 850 30 480 — 9 I 832 772 18 1050 910 47 600 — 10 J 789 739 20 1020 880 41 620 — 11 J 789 739 15 1120 820 — — 900 12 K 808 717 32 1150 900 25 470 — 13 L 839 761 25 1150 870 30 550 — 14 M 842 731 20 1120 880 41 500 — 15 M 842 731 32 1180 890 — — 890 16 M 842 731 15 1120 840 50 300 — 17 N 794 704 20 1120 860 41 450 — 18 N 794 704 25 1150 880 — — 870 19 N 794 704 15 1080 840 52 700 — 20 O 818 750 32 1150 900 26 580 — 21 P 850 749 20 1180 910 41 500 — 22 Q 823 747 15 1120 880 50 550 — 23 R 821 758 30 1080 900 24 470 — Microstructure Martensite- Heating Treatment Austenite Cooling Bainite constituent Cooling Stop Phase Phase Bending Sample Rate Temperature Fraction Fraction Other⁽² Abrasion Work- No. (° C./s) (° C.) (%) (%) (%) Resistance ability⁽³ Class 1 — — 87.8 5.4 12.2(M)  1.5 ∘ Example 2 — — 94.6 7.0 5.4(M) 1.7 ∘ Example 3 — — 93.4 8.1 6.6(M) 1.9 ∘ Example 4 — — 72.1 6.9   27.9(F + P) 1.6 ∘ Example 5 — — 86.0 9.9  14(M) 1.9 ∘ Example 6 — — 90.8 10.6  9.2(M) 2.1 ∘ Example 7 — — 94.1 8.9 5.9(M) 1.8 ∘ Example 8 — — 96.5 9.5 3.5(M) 2.0 ∘ Example 9 — — 80.9 7.3 19.1(M)  1.7 ∘ Example 10 — — 76.2 6.3 23.8(F)  1.6 ∘ Example 11 48 500 94.2 9.6 5.8(M) 1.9 ∘ Example 12 — — 98.9 16.8  1.1(M) 2.6 ∘ Example 13 — — 86.3 8.9 13.7(M)  1.7 ∘ Example 14 — — 99.2 13.9  0.8(M) 2.4 ∘ Example 15 24 600 93.2 10.6  6.8(F)  2.0 ∘ Example 16 — —  2.3 0.2 97.7(M)  2.1 x Comparative Example 17 — — 94.9 11.3  5.1(M) 2.2 ∘ Example 18 28 530 89.0 8.8  11(M) 2.0 ∘ Example 19 — —  2.2 0.1   97.8(F + P) 1.1 ∘ Comparative Example 20 — — 82.4 9.2 17.6(M)  2.1 ∘ Example 21 — — 81.3 0.9 18.7(M)  1.1 ∘ Comparative Example 22 — — 76.6 3.1 23.4(M)  1.3 ∘ Comparative Example 23 — — 84.3 1.5 15.7(M)  1.3 ∘ Comparative Example Note 1: An underlined portion indicates a value out of the range according to the present invention. Note 2: F: ferrite, P: pearlite, M: martensite Note 3: bending workability ∘: without a crack, x: with a crack 

1. A thick steel plate having a chemical composition comprising: C: 0.200% or more and 0.350% or less, by mass %; Si: 0.05% or more and 0.45% or less, by mass %; Mn: 0.50% or more and 2.00% or less, by mass %; P: 0.020% or less, by mass %; S: 0.005% or less, by mass %; Al: 0.005% or more and 0.100% or less, by mass %; and Fe and inevitable impurities, wherein: CI, which is defined by equation (1) below, is 40 or more, CI=60C+8Si+22Mn+10(Cu+Ni)+14Cr+21Mo+15V  (1), in equation (1), atomic symbols respectively denote the contents (mass %) of the corresponding alloy chemical elements, and a chemical element which is not contained is set to be 0, and the thick steel plate has steel microstructure in which (i) an area fraction of a bainite phase is 60% or more and (ii) an area fraction of Martensite-Austenite constituent in the bainite phase is 5% or more and less than 20% with respect to the whole microstructure, and (iii) the remaining constituent phases are at least one of a ferrite phase, a pearlite phase, and a martensite phase.
 2. The thick steel plate according to claim 1, wherein the chemical composition of the thick steel plate further comprises at least one selected from: Cu: 0.03% or more and 1.00% or less, by mass % Ni: 0.03% or more and 2.00% or less, by mass %; Cr: 0.05% or more and 2.00% or less, by mass %; Mo: 0.05% or more and 1.00% or less, by mass %; V: 0.005% or more and 0.100% or less, by mass % Nb: 0.005% or more and 0.100% or less, by mass %; Ti: 0.005% or more and 0.100% or less, by mass %; and B: 0.0003% or more and 0.0030% or less, by mass %.
 3. The thick steel plate according to claim 1, wherein the chemical composition of the thick steel plate further comprises at least one selected from: REM: 0.0005% or more and 0.0080% or less, by mass %; Ca: 0.0005% or more and 0.0050% or less, by mass %; and Mg: 0.0005% or more and 0.0050% or less, by mass %.
 4. The thick steel plate according to claim 2, wherein the chemical composition of the thick steel plate further comprises at least one selected from: REM: 0.0005% or more and 0.0080% or less, by mass %; Ca: 0.0005% or more and 0.0050% or less, by mass %; and Mg: 0.0005% or more and 0.0050% or less, by mass %.
 5. A method for manufacturing a thick steel plate, the method comprising: heating a cast piece or a steel piece to a temperature of 950° C. or higher and 1250° C. or lower, the cast piece or steel piece having a chemical composition comprising: C: 0.200% or more and 0.350% or tens, by mass %; Si: 0.05% or more and 0.45% or less, by mass %; Mn: 0.50% or more and 2.00% or less, by mass %; P: 0.020% or less, by mass %; S: 0.005% or less, by mass %; Al: 0.005% or more and 0.100% or less, by mass %; and Fe and inevitable impurities, wherein CI, which is defined by equation (1) below, is 40 or more, CI=60C+8Si+22Mn+10(Cu+Ni)+14Cr+21Mo+15V  (1), in equation (1), atomic symbols respectively denote the contents (mass %) of the corresponding alloy chemical elements, and a chemical element which is not contained is set to be 0, performing hot rolling with a finishing delivery temperature equal to or higher than Ar₃, and performing accelerated cooling, immediately after the hot rolling has been performed, at a cooling rate of 5° C./sec or more to a temperature range of 400° C. or higher and 650° C. or lower.
 6. The method according to claim 5, wherein the chemical composition of the cast piece or a steel piece further comprises at least one selected from: Cu: 0.03% or more and 1.00% or less, by mass %; Ni: 0.03% or more and 2.00% or less, by mass %; Cr: 0.05% or more and 2.00% or less, by mass %; Mo: 0.05% or more and 1.00% or less, by mass %; V: 0.005% or more and 0.100% or less, by mass %; Nb: 0.005% or more and 0.100% or less, by mass %; Ti: 0.005% or more and 0.100% or less, by mass %; and B: 0.0003% or more and 0.0030% or less, by mass %.
 7. The method according to claim 5, wherein the chemical composition of the cast piece or steel piece further comprises at least one selected from: REM: 0.0005% or more and 0.0080% or less, by mass %; Ca: 0.0005% or more and 0.0050% or less, by mass %; and Mg: 0.0005% or more and 0.0050% or less, by mass %.
 8. The method according to claim 6, wherein the chemical composition of the cast piece or steel piece further comprises at least one selected from: REM: 0.0005% or more and 0.0080% or less, by mass %; Ca: 0.0005% or more and 0.0050% or less, by mass %; and Mg: 0.0005% or more and 0.0050% or less, by mass %.
 9. A method for manufacturing a thick steel plate, the method comprising: heating a cast piece or a steel piece to a temperature of 950° C. or higher and 1250° C. or lower, the cast piece or steel piece having a chemical composition comprising: C: 0.200% or more and 0.350% or less, by mass %; Si: 0.05% or more and 0.45% or less, by mass %; Mn: 0.50% or more and 2.00% or less, by mass %; P: 0.020% or less, by mass %; S: 0.005% or less, by mass %; Al: 0.005% or more and 0.100% or less, by mass %; and Fe and inevitable impurities, wherein CI, which is defined by equation (1) below, is 40 or more, CI=60C+8Si+22Mn+10(Cu+Ni)+14Cr+21Mo+15V  (1), in equation (1), atomic symbols respectively denote the contents (mass %) of the corresponding alloy chemical elements, and a chemical element which is not contained is set to be 0, performing hot rolling, performing air cooling to a temperature lower than 400° C., then performing reheating to a temperature equal to or higher than the Ac₃ and 950° C. or lower, and performing cooling, immediately after the reheating has been performed, at a cooling rate of 5° C./sec or more to a temperature range of 400° C. or higher and 650° C. or lower.
 10. The method according to claim 9, wherein the chemical composition of the cast piece or a steel piece further comprises at least one selected from: Cu: 0.03% or more and 1.00% or less, by mass %; Ni: 0.03% or more and 2.00% or less, by mass %; Cr: 0.05% or more and 2.00% or less, by mass %; Mo: 0.05% or more and 1.00% or less, by mass %; V: 0.005% or more and 0.100% or less, by mass %; Nb: 0.005% or more and 0.100% or less, by mass %; Ti: 0.005% or more and 0.100% or less, by mass %; and B: 0.0003% or more and 0.0030% or less, by mass %.
 11. The method according to claim 9, wherein the chemical composition of the cast piece or steel piece further comprises at least one selected from: REM: 0.0005% or more and 0.0080% or less, by mass %; Ca: 0.0005% or more and 0.0050% or less, by mass %; and Mg: 0.0005% or more and 0.0050% or less, by mass %.
 12. The method according to claim 10, wherein the chemical composition of the cast piece or steel piece further comprises at least one selected from: REM: 0.0005% or more and 0.0080% or less, by mass %; Ca: 0.0005% or more and 0.0050% or less, by mass %; and Mg: 0.0005% or more and 0.0050% or less, by mass %.
 13. The method according to claim 5, wherein the thick steel plate has a steel microstructure in which (i) an area fraction of a bainite phase is 60% or more and (ii) an area fraction of Martensite-Austenite constituent in the bainite phase is 5% or more and less than 20% with respect to the whole microstructure, and (iii) the remaining constituent phases are at least one of a ferrite phase, a pearlite phase, and a martensite phase.
 14. The method according to claim 9, wherein the thick steel plate has a steel microstructure in which (i) an area fraction of a bainite phase is 60% or more and (ii) an area fraction of Martensite-Austenite constituent in the bainite phase is 5% or more and less than 20% with respect to the whole microstructure, and (iii) the remaining constituent phases are at least one of a ferrite phase, a pearlite phase, and a martensite phase. 